Rectangular-section foam blocks are commonly made from a mass of blowable and sinterable plastic particles, especially prefoamed polystyrene beads, by steam in a steam chest mold. The mold can comprise a rectangular inner mold with a plurality of inner mold walls movable from each other and having a multiplicity of processing openings and an outer mold tightly closable and surrounding the inner mold, the outer mold casing having at least one steam inlet and an air venting and vacuum pump connector pipe.
In the known mold the inner mold construction and the outer mold are integrated with each other by constructing the single walls as double walls in such away that the inner mold and the outer mold perform practically no functions independently of each other.
This structure is comparatively expensive because the individual walls must have a sufficient rigidity for a tight seal and seals subject to wear are used between the individual walls.